1. Field of Invention
The present invention relates to bottom layer, thermosetting anti-reflective coating compositions for use in multi-layer photoresist systems, particularly to those having improved etch rates, improved conformality, and improved optical density at 193 nm wavelength exposure.
2. Background of the Prior Art
Bottom layer anti-reflective coating compositions for use in multilayer photoresist systems have traditionally contained high molecular weight thermoplastic binders such as polysulfones, polyurea sulfones, and poly (vinylpyridine) with high insolubility in photoresist solvents, which has also been referred to as "high differential solubility". These binders serve to inhibit intermixing of the anti-reflective composition with the top layer photoresist. Such thermoplastic binders often require strongly polar solvents such as N-methylpyrrolidone, .UPSILON.-butyrolactone, and tetrahydrofurfuryl alcohol, which are hygroscopic, have high surface tensions, and exhibit low volatility. While such solvents may be beneficial to "differential solubility", they also lead to a variety of film defects such as dimpling, dewetting, voiding, bubbling, and thickness variations, because of their low volatility.
There is a present trend to reduce the feature size of semiconductor circuitry. As the feature size approaches sub-0.30 micron dimensions, the aforementioned disadvantages of thermoplastic anti-reflective coatings, as well as the drawbacks described in U.S. Pat. Nos. 5,234,990, 5,401,614, 5,482,817, 5,554,485 and European patent application no. 95480087.6, incorporated herein by reference, become increasingly problematic. The most notable problem is that their so-called resistance to intermixing with photoresists becomes less and less complete. Accordingly, slight intermixing always occurs, producing small but discernible distortions at the bottom of resist features. Because the feature sizes are so small, even these small distortions become unacceptable for producing good quality, practical devices.
In order to overcome these drawbacks, there has arisen a need to develop binders, for anti-reflective coatings, from thermosetting, rather than thermoplastic, polymers. The thermosetting polymers would thermally cure sufficiently to be immiscible in photoresist solvents, yet be coatable from faster drying solvents. Thus solvent resistance and coating quality could both be improved. U.S. Pat. No. 5,693,691, entitled "Thermosetting Anti-Reflective Coatings Compositions", and our co-pending application Ser. No. 08/692,714, entitled "Method for Making Multilayer Resist Structures with Thermosetting Anti-Reflective Coatings" herein incorporated by reference, describe the development of thermosetting anti-reflective coatings and disclose novel improvements in composition and methods. The anti-reflective coatings described therein are comprised principally of an oligomeric, hydroxyl-functional resin, an aminoplast crosslinking agent, a protonic acid catalyst, and an appropriate solvent vehicle, wherein the hydroxyl-functional resin is the reaction product of a phenolic or carboxylic acid dye with a low molecular weight epoxy resin having an epoxy functionality of 3 to 10. The coatings are cured by baking for 30 to 120 seconds at temperatures above 150.degree. C. As taught in U.S. Pat. No. 5,693,691, these compositions, which are soluble in more volatile organic solvents are improvements over the prior art since they offer (1) high optical density in ultra thin films (&lt;2000 .ANG.); (2) virtually no intermixing with photoresists; (3) storage stability in catalyzed form; and (4) commercially feasible synthesis techniques for linking chromophores to an oligomeric resin matrix.
Although the above-described dye-attached thermosetting anti-reflective coatings derived from low molecular weight epoxy resins provide many unexpected benefits, they too have drawbacks. One such drawback occurs upon plasma etching images into the anti-reflective coating layer. For example, in U.S. Pat. No. 5,693,691 (see Example 1), oxygen plasma etching proceeds at rates no faster than about 1.25 times that of prior art thermoplastic resins, such as the polyarylethersulfone anti-reflective coating described in U.S. Pat. No. 5,234,990. Since the anti-reflective coating layer thickness is typically 0.05-0.10 microns, a significant negative etch bias may be observed at resist feature sizes below 0.30 microns unless the plasma etch process is highly anisotropic.
Another limitation of dye-attached thermosetting anti-reflective coatings derived from low functionality epoxy resins is their tendency to planarize substrate topography rather than deposit conformally over surface features. The lack of conformality leads to even greater etch biasing of the photoresist since overetching must be applied to remove the anti-reflective coating from trench structures where it tends to build up during the coating and baking processes.
European patent application no. 94305124.3 discloses thermosetting anti-reflective coatings for deep U.V. exposure which comprise at least one compound (typically a polymer or oligomer) having one or more glycidyl functions, at least one phenolic anthracene dye, and a solvent capable of dissolving these compounds. However, unlike the anti-reflective coating compositions discussed above, the anthracene dye in the title compositions is not attached to the glycidyl-bearing polymer prior to cure and an aminoplast crosslinking agent is not present in the composition. Consequently, heating for several minutes at high temperatures is required to produce sufficient reaction between the phenolic anthracene dye and the glycidyl-bearing polymer to insolubilize the coating. This long cure cycle reduces wafer throughput and makes the process generally unacceptable to manufacturers. In addition, the preparation of the title anti-reflective coatings, particularly that of the phenolic anthracene dye components, involves many steps, making the coatings too expensive to produce and use on a practical basis.
U.S. Pat. No. 5,597,868 discloses thermosetting anti-reflective coatings for 193 nm photolithography which cure analogously to the deep U.V. coatings described in European patent application no. 94305124.3. A polyphenolic dye such as a novolac resin is combined with an acrylic polymer which has pendant epoxide functionality. Heating the coating results in a thermosetting reaction between the phenolic hydroxyl groups of the dye and the epoxide groups of the polymer. As claimed therein, however, the curing process must proceed for more than 10 minutes at temperatures greater than 170.degree. C. to be effective.